Hans Eysenck's theory is based primarily on physiology and genetics. Although he was a behaviorist who considered learned habits of great importance, he considers personality differences as growing out of our genetic inheritance. He is, therefore, primarily interested in what is usually called temperament.

Temperament is that aspect of our personalities that is genetically based, inborn, there from birth or even before. That does not mean that a temperament theory says we don't also have aspects of our personality that are learned, it's just that Eysenck focused on "nature," and left "nurture" to other theorists.

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Extraversion/Introversion : Extraversion is characterized by being outgoing, talkative, high on positive affect (feeling good), and in need of external stimulation. According to Eysenck's arousal theory of extraversion, there is an optimal level of cortical arousal, and performance deteriorates as one becomes more or less aroused than this optimal level. Arousal can be measured by skin conductance, brain waves or sweating. At very low and very high levels of arousal, performance is low, but at a more optimal mid-level of arousal, performance is maximized. Extraverts, according to Eysenck's theory, are chronically under-aroused and bored and are therefore in need of external stimulation to bring them up to an optimal level of performance. Introverts, on the other hand, are chronically over-aroused and jittery and are therefore in need of peace and quiet to bring them up to an optimal level of performance.

Neuroticism/Stability : Neuroticism or emotionality is characterized by high levels of negative affect such as depression and anxiety. Neuroticism, according to Eysenck's theory, is based on activation thresholds in the sympathetic nervous system or visceral brain. This is the part of the brain that is responsible for the fight-or-flight response in the face of danger. Activation can be measured by heart rate, blood pressure, cold hands, sweating and muscular tension (especially in the forehead). Neurotic people, who have low activation thresholds, and unable to inhibit or control their emotional reactions, experience negative affect (fight-or-flight) in the face of very minor stressors - they are easily nervous or upset. Emotionally stable people, who have high activation thresholds and good emotional control, experience negative affect only in the face of very major stressors - they are calm and collected under pressure.

The two dimensions or axes, extraversion-introversion and emotional stability-instability, define four quadrants. These are made up of:

Further research demonstrated the need for a third category of temperament:[2]

Psychoticism/Socialisation :
Psychoticism is associated not only with the liability to have a psychotic episode (or break with reality),
but also with aggression. Psychotic behavior is rooted in the characteristics of toughmindedness,
non-conformity, inconsideration, recklessness, hostility, anger and impulsiveness. The physiological
basis suggested by Eysenck for psychoticism is testosterone, with higher levels of psychoticism associated
with higher levels of testosterone.

The following table describes the traits that are associated with the three temperaments in Eysenck's model of personality:

In 1985 a revised version of EPQ was described—the EPQ-R—with a publication in the journal Personality and Individual Differences.[4]
This version has 100 yes/no questions in its full version and 48 yes/no questions in its short scale version.
A different approach to personality measurement developed by Eysenck, which distinguishes between different facets of these traits, is the Eysenck Personality Profiler.